When members of the Bradford & Bingley Building Society voted for it to become a bank (owned by its shareholders), rather than remaining a building society (owned by its members, i.e. those who held accounts with it), account holders became entitled to receive shares in that bank. It was known that many such account holders would want to cash in their nice little windfall straight away, so it was rather pointless sending out share certificates to everyone, as many new shareholders would then have to pay a broker to sell their shares for them.
So initially all of the new shares in the bank were placed into the Bradford and Bingley Nominee Account, with shareholders simply being sent a statement of their right to either cash them in straight away (without the need to employ a broker) or to receive an actual share certificate. Alternatively they could simply leave them in the nominee account and take action later on.
Well, that's how I read/interpret it anyway, having just waded through this lot!
http://consumer.bbg.co.uk/~/media/Files/B/Bradford-and-Bingley-V2/subordinated-debt/psb-listing-particulars.pdf